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Why do fluorescent lights buzz?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

July 22, 2003

Dear Straight Dope:

Why do fluorescent light fixtures buzz? Are the lights going bad, or is it the fixture itself? Is it a sign someone has placed monitoring devices in my house? Is there any danger involved--say, a potential electrical short, or an explosion? Or, is it perhaps an attempt to communicate with me from outside the atmosphere? Inquiring minds want to know.

Jon Jackson

Q.E.D. replies:

Let’s start with a bit of history. General Electric introduced fluorescent lamps to the public in 1939 at the New York World’s Fair. ("Myrtle! Get a load of this! Fluorescent lights!" Oh, for those simpler days.) Invented in 1926 by a German team consisting of Friedrich Meyer, Hans Spanner and Edmund Germer, whose patent GE purchased for $180,000, fluorescent lamps quickly became popular in industrial and commercial buildings due to their long operating life and low cost-per-lumen (a lumen is a measure of light output). However, they failed to gain much popularity among residential consumers, at least until fairly recently, due to the loud hum or buzz they sometimes produced, the blinking they did when first turned on, and the unnatural color balance of the light they emitted.

What causes the buzz? Basically a fluorescent lamp is a glass tube filled with an inert gas and some mercury. Filaments in the ends of the tube are turned on briefly by the starter to vaporize the mercury. Meanwhile, a voltage is applied across the tube to ionize the mercury vapor, allowing a current to flow. The ionized vapor emits ultraviolet light in response to the electrical current, which in turn causes the material lining the inside of the tube, called a phosphor (actually a mixture of several phosphors), to fluoresce, i.e., emit visible light. Don’t worry, we’re getting to the buzz. Once the gas in the tube is ionized, the current through it rises so rapidly that, if it were allowed to continue unchecked, it would reach dangerous levels and trip the circuit breaker. To avoid that, a device known as a ballast is used to limit the current through the tube. The ballast is your culprit here.

Prior to the introduction of quieter, more efficient electronic units, a ballast was an electromagnetic device called an inductor. An inductor consists of many turns of wire wound around an iron core and operates, as one might suppose, by the principle of induction. A change in the flow of alternating current through the inductor creates a changing magnetic field (ever built an electromagnet by winding a lot of wire around a bolt or nail? Same idea), which in turn creates, or induces, a changing voltage in the wire, retarding the current flow. The upshot is that the inductor acts as a sort of electrical brake, limiting the amount of juice passing through the tube.

Now here’s the rub: Under certain conditions, the magnetic field can cause an effect called magnetostriction, which means that the magnetic field physically squeezes the core, altering its shape slightly. Since the fluorescent fixture is operating on alternating current at 60 Hz (that is, 60 Hertz, or cycles per second in the U.S.; 50 Hz in many other countries), the core gets squeezed and released at twice that rate, or 120 Hz, resulting in the buzz you often hear. That’s it, really. No eavesdropping, no attempts at extraterrestrial communication, nothing to worry about. Just don’t start talking back.

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STAFF REPORTS ARE WRITTEN BY THE STRAIGHT DOPE SCIENCE ADVISORY BOARD, CECIL'S ONLINE AUXILIARY. THOUGH THE SDSAB DOES ITS BEST, THESE COLUMNS ARE EDITED BY ED ZOTTI, NOT CECIL, SO ACCURACYWISE YOU'D BETTER KEEP YOUR FINGERS CROSSED.